Various currently used apparatus are known in this field.
In the first place, there is an elevator constituted by a rotating Archimede's screw with a cylinder whose diameter is equal to that of the screw.
An apparatus of this type is bulky, difficult to handle and sucks in a large quantity of water at the same time as the fish. Beyond an angle of 45.degree. with respect to the level of the water, this screw pump runs dry and it is therefore necessary that the cylinder is very long in order to reach a satisfactory useful height. The intake of fish is manual in order to prevent excessively fast rotation of the screw.
A bucket elevator is also known constituted by a conveyor belt provided with battens: it does not operate beyond an angle of 45.degree., must be operated manually, is heavy and bulky.
One also uses a pump operating at very low pressure and with a high output, a so-called trout pump. Positioning of such a pump requires a considerable amount of time, since it is necessary to install large diameter pipes whilst taking care to reduce pressure losses in the pipe, as far as possible. The fish instinctively fighting against the current will thus attempt to move away from the low pressure suction zone, to which it must be returned by a net constantly reducing the area in which it is able to swim. It will thus undergo traumatisms which will hinder its ultimate development or simply reduce the quality of its flesh if it is eaten shortly after this pumping operation. In any case, the arrival of fish in the pump will be irregular and will take place in surges.